The accumulation of frozen contaminants (i.e., frost, snow, ice) and other debris (i.e., dirt, dust, insect residue) on an aircraft can have a significant impact on the safety and operation of the aircraft. Likewise, during wind-tunnel testing, the buildup of frost on aircraft wings can compromise the validity of the test and affect the accuracy of the test results requiring that the frost be removed and testing restarted.
For in-service aircraft such as commercial airliners, the accumulation of frost, snow or ice on the aircraft wings can significantly alter the lift and drag characteristics and can increase stall speed, reduce controllability and alter other flight characteristics. In addition to adding weight to the aircraft, frost and ice formations on wings can increase the surface roughness in critical areas such as on the wing leading edges and upper surfaces and can result in a reduction in lift of up to 30% and an increase in drag of up to 40%.
Ice buildup on control surfaces such as rudders, elevators and ailerons can have a significant impact on aircraft performance and controllability. In addition, the buildup of snow, frost and other types of debris such as insect residue on external instrumentation sensors such as pitot/static ports and angle-of-attack indicators can cause inaccurate readouts of flight instrumentation critical to safe flight.
In-flight aircraft are subject to a variety of atmospheric conditions that can result in accumulations of ice on the aircraft. Factors such as ambient temperature, aircraft surface skin temperature, relative humidity and air speed can influence whether or not ice, frost or snow accumulates. For example, precipitation occurring at ambient temperatures below freezing can remain in a supercooled liquid state (i.e., freezing rain) and can accumulate on the leading edge and upper surfaces of an aircraft wing causing surface roughness which can significantly alter the aircraft flight characteristics.
Parked aircraft and aircraft undergoing ground operations (i.e., taxiing, re-fueling, passenger loading) are also susceptible to accumulations of frozen contaminants. For example, residual ice from a previous flight can remain on wing leading edges and other control surfaces of the aircraft. Under high humidity conditions, frost can accumulate on aircraft during overnight ground storage.
Unfortunately, frost and other ice formations are difficult to detect visually due to the translucent or nearly-translucent nature of such formations. Clear ice is likewise difficult to detect other than by tactile means. Such clear ice can form on the upper surfaces of wings in areas adjacent the fuel tanks of aircraft that have recently landed after high altitude operation. The fuel tanks of such aircraft may contain super-cooled fuel as a result of the high altitude operation and can cause rain, drizzle or high humidity to freeze upon contact with the outer wings surfaces in areas adjacent to the fuel tanks.
In light of the safety issues raised by accumulation of frozen contaminants on aircraft, the Federal Aviation Administration (FAA) implemented a “clean aircraft” concept as currently defined by Federal Aviation Regulations (FAR) for airline operations. In general, the clean aircraft concept prohibits take-off of an aircraft when snow, ice or frost adheres to wings, control surfaces and other components of the aircraft. As part of the clean aircraft concept, FAR regulations require operators to install ground deicing/anti-icing equipment during winter operations. Deice materials typically comprise heated solutions of freezing point depressant (FPD) fluids which are applied to areas of the aircraft suspected of having accumulations of frozen contaminants. Anti-icing fluids are typically thicker and have a lower freezing point in order to provide a protective film for delaying reformation of frozen contaminants.
Despite deicing and anti-icing measures, detection of the accumulation of frozen contaminants such as snow, ice, and frost on an aircraft or other vehicle is difficult. Clear ice is difficult to detect visually regardless of its thickness and may not be detectable other than by tactile inspection by personnel with the use of special equipment such as man-lifts that are required to access wings and control surfaces of larger aircraft. Furthermore, the use of such special equipment presents the potential for damage to the aircraft and injury to personnel under the types of weather conditions that are conducive to the formation of ice.
Because tactile inspection of in-flight aircraft surfaces is impossible and considering the above-described difficulties associated with visual detection of frozen contaminants, it can be seen that there exists a need in the art for a system and method for detecting and warning of the presence or absence of frozen contaminants and other debris (i.e., dirt, dust and insect residue) in order to improve the safety and operation of aircraft and other vehicles and structures. Furthermore, there exists a need in the art for a system and method for detecting the existence of frozen contaminants and other debris on ground-based vehicles such as wind tunnel models in order to reduce the loss of time and expense associated with undetected frost buildup.
Additionally, there exists a need in the art for a system and method for detecting films on surfaces that does not rely on visual or tactile inspection and which has the capability for distinguishing between the composition of the contaminant (i.e., insect debris vs. ice accumulation). Finally, there exists a need in the art for a system and method for detecting films on surfaces which is of low cost, simple in construction and reliable in operation.